1. Field of the Invention
The present invention relates generally to shaft seals and, more particularly, is concerned with a method of forming a coating on seal surface in a nuclear reactor coolant pump.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 550 degrees F. and 2500 psi. The pump basically includes three general sections from bottom to top--hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem sealing assemblies--lower primary, middle secondary and upper tertiary sealing assemblies. The sealing assemblies are located concentric to, and near the top end of, the pump shaft. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor coolant system from leakage along the pump shaft to the containment atmosphere during normal operating condition. Representative examples of pump shaft sealing assemblies known in the prior art are the ones disclosed in U.S. Pat. No. 3,522,948 to MacCrum, Singleton U.S. Pat. No. 3,529,838, Villasor U.S. Pat. No. 3,632,117, Andrews et al U.S. Pat. No. 3,720,222, Boes U.S. Pat. No. 4,275,891, Jenkins U.S. Pat. No. 4,690,612 and Quinn U.S. Pat. No. 4,693,481 and in the first three patent applications cross-referenced above, all of which are assigned to the same assignee as the present invention.
Historically, the pump shaft seals constitute the main problem area for the reactor coolant pumps and significantly contribute to the utilization factor in nuclear power plants. The seals must be capable of breaking down the high system pressure (about 2500 psi) safely. The tandem arrangement of the three seals is used to break down the pressure, with the lower primary seal absorbing most of the pressure drop (approximately 2250 psi). The lower primary sealing assembly is the main seal of the pump. It is typically a hydrostatic, "film-riding", controlled leakage seal whose primary components are an annular runner which rotates with the pump shaft and a non-rotating seal ring which remains stationary with the pump housing. Whereas the components of the lower primary sealing assembly are not intended to contact or rub together, corresponding components of the middle and upper sealing assemblies, a rotating runner and non-rotating seal ring, provide contacting or rubbing seals.
Heretofore, the runner components of the rub-type sealing assemblies (the middle secondary and upper tertiary sealing assemblies) have been composed of a stainless steel substrate having an outer coating of chromium carbide on the surface of the runner components which rubs against the seal ring. The coating is formed by depositing chromium carbide powder on the stainless steel substrate using a detonation gun technique. Bonding between the coating and the substrate is achieved purely by mechanical impact forces when the powdered chromium carbide is impinged onto the substrate. The density of the coating thus applied is typically less than 100% of theoretical.
The chromium carbide coating thus formed has proven to be less than satisfactory. Blistering has been observed to occur on the chromium carbide coated runners. The blistering is caused by contact with corrosive materials making up the nuclear water chemistry employed in nuclear reactors, such as chlorine or sulfur bearing compounds. These corrosive materials penetrate through the pores of the chromium carbide coating to the stainless steel/coating interface. Hydrogen gas formation caused by the corrosive mechanism then results eventually in a spalling, or blistering, of the coating's surface. Thus, the blistering is attributed to the inherent porosity heretofore present in the coating and the lack of optimum bonding at the stainless steel/coating interface.
One recent approach to improving the chromium carbide coating is disclosed in the last patent application cross-referenced above, also assigned to the same assignee as the present invention. After the molten powder is deposited as a coating on the stainless steel substrate by using the detonation gun, it is encased and hot isostatic pressing (HIP) is applied in order to densify the coating to substantially its full theoretical density. In the HIP processing step, the coating is subjected to a high pressure-temperature cycle which eliminates pores and produces a sintering of the coating for providing a metallurgical bonding between constituent materials of the coating. However, the highest temperature reached during the HIP processing step is generally not more than two-thirds of the melting points of constituent materials of the coating so that release of gases within the encased coating is avoided.
Even though this approach represents a step in the right direction toward eliminating pores in the applied coating and thereby reducing blistering, the use of the detonation gun technique to deposit the coating to the substrate imposes limitations on the quality of the bonding achieved between the coating and substrate. The powder which will form the coating is a mixture of a ceramic and a metal binder which both must be in a molten state to be sprayed using the detonation gun technique. The detonation gun applies the molten mixture in splats which cool very fast once they deposit on the substrate. The cooled splats produce a coating which has suboptimal interparticle bonding and is susceptible to microcracking and delamination in a thermal cyclical environment.
The subsequent application of hot isostatic pressing to the deposited chromium carbide coating densifies the coating but generally fails to rectify the problems created by use of the detonation gun. Consequently, a need exists for improvement of the above-described recent approach so that further improvement of the applied coating can be realized.